Fuel cells are electrochemical devices that directly convert chemical energy in fuels into electric energy, and in the fuel cells the fuels of reductants such as reformed hydrogen manufactured from hydrogen, methanol or fossil fuel are electrochemically oxidized by oxidants such as air or oxygen. They draw attention recently and are expected to be “clean” or relatively little pollutant producing sources of electric energy that provide power in higher conversion efficiency than internal engines with superiority in stillness and generating lower pollutant such as NOx, SOx and particulate matter (PM) causing air pollution. They are, for instance, expected to operate in replacement of power systems of the conventional automobiles, and as thermal and electric power providing systems and dispersed electric power sources for such as residences.
The most common classification of fuel cells is by the type of electrolyte used in the cells and includes alkaline fuel cell (AFC), phosphoric acid fuel cell (PAFC), molten carbonate fuel cell (MCFC), solid oxide fuel cell (SOFC) and polymer electrolyte fuel cell (PEFC). The PAFC and PEFC using proton conductivity electrolyte can operate at high efficiency without suffering the thermodynamic limitation by Carnot cycle, and it attains the theoretical efficiency of 83% at the temperature of 25° C. The PEFC, especially, attract attention because they are useful for power sources for lower pollutant automobiles and high efficiency power generating systems, with improvement in its performance by recent development in electrolyte membrane and catalyst technologies.
The PEFC has a structure which is provided with a planar or cylindrical, for example, polymer electrolyte layer and a gas diffusion electrode on each side of the polymer electrolyte layer, a catalyst layer being therebetween on each side, and that kind of the structure is called a membrane electrode assembly (MEA) and the conventional PEFC is constituted of a plurality of the MEAs stacked and separators disposed therebetween. Through the aforementioned gas diffusion electrode, fuel gas and air reach the surfaces of the electrolyte layer and the catalyst layer, and the electrode is required to have superiority in gas diffusiveness and electric conductivity for obtaining the generated electric current. Carbon fiber paper (namely, carbon paper) is generally used for conventional gas diffusion electrodes. Instead, suggested are, for example, carbon fiber cloth (namely, carbon cloth, disclosed, for instance, in Patent Document 1), unwoven fabrics including carbon nanofiber and/or carbon nanocone (disclosed, for instance, in Patent Document 3), or unwoven fabrics impregnated with conductive material (disclosed, for instance, in Patent Document 4).    Patent Document 1: JP 07-105957 A    Patent Document 2: JP 08-007897 A    Patent Document 3: JP 2005-149745 A    Patent Document 4: JP 2000-513480 A